Soils 205-90
Lecture 8- Clay mineral properties
Video sessions Pages in text.
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A. 1:1 Type Minerals
1. Mostly, kaolinite
2. Unit layers H-bonded together
è "fixed lattice type"
ü no interlayer activity
ü no shrink-swell
ü only external surface
3. Well crystallized
(a) little isomorphous substitution
ü low cation adsorption
(b) larger particle size (0.1 - 5 m m)
- hexagonal shaped
B. 2:1 Type Minerals
1. Expanding lattice
(a) Smectite group (mostly, montmorillonite)
(b) Freely expanding
¯ water in interlayer
= large shrink-swell
¯ adsorbed cations in interlayer
- offset the isomorphous substitution
¯ large internal surface
(c) Poorly crystallized
¯ small size
¯ isomorphous substitution
= large cation adsorption
(d) Vermiculite
¯ similar to smectites
except Al+3 for Si+4 in tetrahedral layer
¯ interlayer ions are more structured (Mg+2 + H2O)
= limited expansion
¯ large cation adsorption
2. Non-expanding lattice
(a) Fine-grained micas or illite
u Al+3 and K+ substitute for Si+4 (tetrahedral sheet)
u weathering at edges = release of K+
- very limited expansion
- medium cation adsorption
- limited internal surface
- properties between kaolinite and vermiculite
(b) Chlorites
u Mg-octahedral sheet replace K+ of illite
u properties similar to illite
3. 2:1 structure comparisons
C. Summary of Properties
Surface Area (m2/g) Interlayer Cation Size (mm) External Internal Spacing (nm) Sorption Kaolinite 0.1-5.0 10-50 - 0.7 5-15 Smectite <1.0 70-150 500-700 1.0-2.0 85-110 Vermiculite 0.1- 5.0 50-100 450-600 1.0-1.4 100-120 Illite 0.1-2.0 50-100 5-100 1.0 15-40 Humus coatings - - - 100-300
D. Clay Genesis and Distribution
1. Stages of weathering
(a) alkali metals and alkaline earths dissolve
(Na+, K+, Ca+2, Mg+2)
(b) Si dissolves and leaches
(c) continual reforming of new clay minerals
2. Clays reflect weathering processes
Young, weakly weathered soils
= fine-grained mica, chlorite, vermiculite
Intermediate weathering
= vermiculite, smectite, kaolinite
Strong weathering
= kaolinite, hydrous oxides
